Have you ever held an actual lump of coal in your hands? It looks like a black rock. But the thing you immediately notice is its weight. Coal feels incredibly light compared to a normal rock. That's because coal is composed largely of carbon, which is the same lightweight element found in carbon fiber tennis rackets and bike frames.

This simple black rock is surprisingly important to the United States. The carbon is what makes coal such a valuable source of energy. When the carbon combines with oxygen, it gives off heat. That heat is very handy for producing electricity. In the U.S., we burn more than two trillion pounds of coal every year to keep the power grid humming. About half of the electricity in the United States currently comes from coal.

Where does all the coal come from? We mine it out of the ground using three different techniques:

traditional underground mining

surface mining

mountaintop mining

Underground mining is what we traditionally think of when we think of "coal mining". Miners dig tunnels down into the earth to find coal seams. A seam is layer of coal that can be hundreds of feet deep. Using automated equipment, miners then start digging caverns or rooms into the seam, leaving behind large pillars of coal to support the roof. This is called room and pillar mining. As the miners dig, a conveyor system carries the coal back to the surface.

Once the miners have gone as far as they can with the room and pillar approach, they may begin taking out the pillars, starting at the back of the mine. As the coal in the roof collapses, the miners dig it out in a process called retreat mining.

More than half of the coal mined worldwide comes from underground mining. However, working in the mines is not one of the safest ways to make a living. Mines collapse, and there are also problems with mine gases (which can suffocate miners or explode) and coal dust (which causes black lung).

Coal seams are often close to the surface rather than deep underground. In that case, it is usually faster, safer and more economical to dig the coal out from the top. This process is often referred to as strip mining, although pit mines are also possible.

Strip mining is faster than underground mining because miners can use lots more equipment. And the equipment is much larger than the equipment you see in an underground mine. After peeling away the layer of soil over the coal, miners start blasting and digging out the coal seam. Immense excavators shovel out the coal, or gigantic rotating disks lined with buckets chew through the coal at a rate of tons per second. Conveyor systems or the biggest dump trucks in the world haul the coal away. Each truck can carry hundreds of tons of coal per trip.

One of the newer and more controversial techniques in coal mining is called mountaintop mining. It is common in West Virginia. This is one of the least popular techniques from an aesthetic standpoint because miners literally blast off the top of a mountain and then carry away the coal underneath. Hundreds of mountains have disappeared because of this practice. Once the mining is done, miners cover the base of the mountain with dirt and replant with trees. It may take a decade or more for the area to recover.

If the United States digs out two trillion pounds of coal every year, you might be wondering how long the supply can last. It turns out that underground coal deposits are very common in the U.S. It is thought that there might be 500 trillion pounds of more left to mine - enough to last for hundreds of years at the current rate of consumption.

Given that coal is so plentiful, so inexpensive to mine and so widely used, it seems like coal is the ideal energy source. The big problem is the pollution that comes from burning coal. There is direct pollution in the form of things like ash, sulfur and radiation. And then there is the greenhouse gas effect of all the carbon dioxide that burning coal emits. A concept called clean coal technology tries to address these concerns. Clean coal involves everything from scrubbing sulfur out of smoke stack gases to capturing all the carbon dioxide and burying it underground or in the ocean. We will have to see whether these newer technologies take root, or whether coal is replaced over time by greener technologies like solar and wind power.